When is your baby actually due?

ACOG redefined term pregnancy categories in 2013. Early term is 37-38 weeks. Full term is 39-40 weeks. Late term is 41 weeks. Post-term is 42 weeks and beyond. Outcomes improve significantly between 37 and 39 weeks, which is why elective deliveries before 39 weeks without medical indication are discouraged.

IVF due dates are calculated from a known fertilisation event. From egg retrieval: add 266 days. From a day-3 embryo transfer: add 263 days. From a day-5 blastocyst transfer: add 261 days. Because the fertilisation date is precisely known, IVF due dates eliminate the cycle-length guesswork inherent in LMP calculations.

Yes. The standard Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently 35 days, ovulation occurs around day 21, meaning your conception happened about a week later than assumed. The calculator adjusts for this by adding the difference between your cycle length and 28 days. For women with irregular cycles, the LMP method is less reliable and an early ultrasound dating scan is more accurate.

The NHS recommends a dating scan between 10 and 14 weeks, ideally at 11-13 weeks. Ultrasound measurement of the foetal crown-rump length (CRL) at this stage is the most accurate way to confirm gestational age. If there is a discrepancy of more than 5-7 days between the scan date and the LMP date, the scan date takes precedence for all future pregnancy milestones.

A pregnancy is considered post-term at 42 completed weeks (294 days). ACOG and NICE both recommend offering induction of labour between 41 and 42 weeks because placental function begins to decline after 40 weeks and the risk of stillbirth rises slightly with each additional week. Around 10% of pregnancies reach 42 weeks without spontaneous labour. Your midwife or obstetrician will discuss options with you as your due date approaches.

Naegele's rule (LMP + 280 days) has been the standard since 1812 and remains widely used. Its main limitation is the assumption of a uniform 28-day cycle. A 2020 Cochrane review confirmed that ultrasound dating in the first trimester is more accurate than LMP-based dating and reduces post-term inductions. Many NHS trusts now use the early scan date as the definitive due date when it differs materially from the LMP estimate.

Stress and anxiety do not change the calculated due date, but they may influence the timing of labour onset. Conditions such as pre-eclampsia, gestational diabetes, or foetal growth restriction may lead to medically indicated early delivery before the due date. If you have any pre-existing conditions, discuss how they might affect your pregnancy timeline with your midwife or consultant.

The traditional trimester split (weeks 1-12, 13-26, 27-40) is a rough guide. The first trimester ends at the close of week 12, the second at the close of week 26, and the third from week 27 to birth. Some clinicians now use slightly different breakpoints (e.g. ending the first trimester at 13+6 weeks). The NHS uses the three-trimester model for routine screening and appointment scheduling.

The Mittendorf-Williams rule is a refinement of Naegele's rule published in 1990. It found that first-time mothers (nulliparous) carry on average 15 days beyond LMP + 265 days, while subsequent pregnancies are shorter. It also adjusts for maternal age and pre-pregnancy weight. Despite its greater accuracy in some studies, Naegele's rule remains more widely used because it is simpler and ultrasound dating has largely superseded formula-based methods for precision.